In our attempts to purify the soluble cytolytic agent that is released by platelets during clotting, we have found that there is apparent heterogeneity attributable to the fact that the cytolysin binds firmly to several different plasma proteins. Thus, resolution procedures carried out on ion-exchange columns or by PAGE yield several fractions with lytic activity, though much of it appears coincident with serum albumin. Gel-filtration on Sephadex G-10 or G-25 results, as expected, in the appearance of a single sharp peak of protein because of the lack of any significant resolution, and coincident with this peak is a peak of lytic activity. However, additional activity appears at a point where retained molecules would be expected; and the level of activity appearing here is surprisingly high. When material from this second peak--Mr is less than 10,000--is mixed with nonlytic plasma protein, lytic activity again appears in two peaks indicating that some of the low molecular weight activity has become associated with larger protein molecules. Consistent with this view that active molecules are bound to or inhibited by larger protein molecules, we have found striking increases in lytic activity after treating serum with proteolytic enzymes (trypsin or pepsin) or heat (80~ to 100~C for 30 to 120 min). When material treated in this way is applied to Sephadex G-10, there is a marked increase in the activity of the second peak indicating release of a small active molecule. In our studies on the nature of the cells responsible for immunologically mediated damage to tumors, we have found that both Lyt-1 and Lyt-2 lymphocytes are active in the destruction of tumor cells in vitro. However, when mice are actively immunized with cells that bear no MHC disparate antigens but multiple non-MHC H antigens, the recipients become resistant to all tumors bearing the same non-MHC antigens irrespective of the kind of MHC antigens that they possess. There is, in effect, an apparent lack of MHC restriction. The significance of this curious finding remains to be determined. Specific objectives for the coming year are: (a)\to determine whether or not platelets play an important role in antibody-mediated damage to tumors in vivo; (b)\to determine the extent to which tumor cells are shielded from the action of platelets and other effectors of immunologically mediated damage by the blood vessels of the tumor; and (c)\to continue to purify and characterize the soluble cytolysin released from platelets. (IP)